1. Field of the Disclosure
This invention pertains to a method for preparing a relief printing form from a photosensitive element, and in particular, to a method of preparing the relief printing form by imagewise exposing the photosensitive element in a specific environment, and then treating with a washout solution.
2. Description of Related Art
Flexographic printing plates are widely used for printing of packaging materials ranging from corrugated carton boxes to cardboard boxes and to continuous web of plastic films. Flexographic printing plates are used in relief printing in which ink is carried from a raised-image surface and transferred to a substrate. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,759. Photosensitive elements generally have a solid layer of the photopolymerizable composition interposed between a support and a coversheet or a multilayer cover element. Flexographic printing plates are characterized by their ability to crosslink or cure upon exposure to actinic radiation. The plate is imagewise exposed with actinic radiation through an image-bearing art-work or a template, such as a photographic negative or transparency (e.g., silver halide films) for so called analog workflow, or through an in-situ mask having radiation opaque areas that had been previously formed above the photopolymerizable layer for so called digital workflow. The actinic radiation exposure is typically conducted with ultraviolet (UV) radiation. The actinic radiation enters the photosensitive element through the clear areas and is blocked from entering the black or opaque areas of the transparency or in-situ mask. The areas of the photopolymerizable layer that were exposed to the actinic radiation crosslink and harden and/or become insoluble to solvents used during development. The unexposed areas of the photopolymerizable layer that were under the opaque regions of the transparency or the in-situ mask during exposure do not hardened and/or remain soluble. The unexposed areas are removed by treating with a washout solution or heat leaving a relief image suitable for printing. If treated with washout solution, the plate is dried. Although thermal treatment to remove unexposed areas from the layer of the photopolymerizable material advantageously avoids time consuming drying step, washout treatment with solution is a well-accepted commercial practice in the industry to form the relief surface for the printing form. After all desired processing steps, the plate is then mounted on a cylinder and used for printing.
Solid plate preparation workflows involve making an intermediate, i.e., the photographic negative or phototool. Analog workflow requires the preparation of the phototool, which is a complicated, costly and time-consuming process requiring separate processing equipment and chemical development solutions. In addition, the phototool may change slightly in dimension due to changes in temperature and humidity. The same phototool, when used at different times or in different environments, may give different results. Use of a phototool also requires special care and handling when fabricating flexographic printing forms to ensure intimate contact is maintained between the phototool and plate. In particular, care is required in the placement of both the phototool and the plate in the exposure apparatus along with special materials to minimize air entrapment during creation of a vacuum to ensure intimate contact. Additionally care must be taken to ensure all surfaces of the photopolymer plate and phototool are clean and free of dust and dirt. Presence of such foreign matter can cause lack of intimate contact between the phototool and plate as well as image artifacts.
An alternative to analog workflow is termed digital workflow, which does not require the preparation of a separate phototool. Photosensitive elements suitable for use as the precursor and processes capable of forming an in-situ mask in digital workflow are described in U.S. Pat. No. 5,262,275; U.S. Pat. No. 5,719,009; U.S. Pat. No. 5,607,814; U.S. Pat. No. 6,238,837; U.S. Pat. No. 6,558,876; U.S. Pat. No. 6,929,898; U.S. Pat. No. 6,673,509; U.S. Pat. No. 5,607,814; U.S. Pat. No. 6,037,102; and U.S. Pat. No. 6,284,431. The precursor or an assemblage with the precursor includes a layer sensitive to laser radiation, typically infrared laser radiation, and opaque to actinic radiation. The infrared-sensitive layer is imagewise exposed with laser radiation whereby the infrared-sensitive material is removed from, or transferred onto/from a superposed film of the assemblage, to form the in-situ mask having radiation opaque areas and clear areas adjacent the photopolymerizable layer. The precursor is exposed through the in-situ mask to actinic radiation in the presence of atmospheric oxygen (since no vacuum is needed). Furthermore, due in part to the presence of atmospheric oxygen during imagewise exposure the flexographic printing form has a relief structure that is different from the relief structure formed in analog workflow (based upon the same size mask openings in both workflows). Digital workflow creates a raised element (i.e., dot or line) in the relief structure having a surface area of its uppermost surface (i.e., printing surface) that is significantly less than the opening in the in-situ mask corresponding to the relief structure. Digital workflow results in the relief image having a different structure for dots (i.e., raised surface elements) that is typically smaller, with a rounded top, and a curved sidewall profile, often referred to as dot sharpening effect. Dots produced by analog workflow are typically conical and have a flat-top. The relief structure formed by digital workflow results in positive printing properties such as, finer printed highlight dots fading into white, increased range of printable tones, and sharp linework. As such, the digital workflow because of its ease of use and desirable print performance has gained wide acceptance as a desired method by which to produce the flexographic printing form. But not all end-use applications view this dot-sharpening effect as beneficial.
It is known by those skilled in the art that the presence of oxygen (O2) during exposure in free-radical photopolymerization processes will induce a side reaction in which the free radical molecules react with the oxygen, while the primary reaction between reactive monomer molecules occurs. This side reaction is known as inhibition (i.e., oxygen inhibition) as it slows down the polymerization or formation of crosslinked molecules. Many prior disclosures acknowledge that it is desirable for photopolymerization exposure to actinic radiation to occur in air (as is the case for digital workflow), under vacuum (as is the case for analog workflow), or in an inert environment. Oftentimes, nitrogen is mentioned as a suitable inert gas for the inert environment. The implication is that the nitrogen environment is one that contains substantially less than atmospheric oxygen to the extent that all oxygen is removed, or something less than about 10 ppm of oxygen. Nitrogen with oxygen impurity concentration level less than 10 ppm is readily commercially available.
The effect of oxygen associated with digital workflow can impact the ability to hold solid screen patterns in solid printing areas of the relief printing form. It is often desirable for an image that is printed by flexographic relief printing form to increase the density of ink in solid areas of the image, so-called solid ink density. Solid screening is a well-known process for improving the solid ink density in flexographic printing, and is described for instance in U.S. Pat. No. 6,492,095; and U.S. Patent Publication US 2010/0143841. Solid screening consists of creating a pattern in the solid printing areas which is small enough that the pattern is not reproduced in the printing process (i.e., printed image), and large enough that the pattern is substantially different from the normal, i.e., unscreened, printing surface. Often these screening patterns are features in the range of 5 to 30 microns. The inhibition effect of the atmospheric oxygen during imagewise exposure of a photosensitive element for relief printing can result in pattern features being reduced in size by about 15 microns on each edge. Consequently this reduction in feature size, a 30 micron feature will be reduced to a 0 micron feature size for example, limits relief printing form to print increased solid ink densities.
There is a desire to provide printing forms with a relief structure that can hold microcell patterns in solid areas and having relief features similar to analog, that is, raised features that are conically shaped and have a flat-top or substantially flat-top surface, including fine raised printing features of about 5 to 30 microns, such as highlight halftone dots and lines. Each of raised print features of the relief surface includes a top surface area which is the ink carrying surface, a side-wall surface area, and a shoulder surface area which is the transition between the top surface area and the side-wall surface area. For each raised feature the total printing area capable of contacting the substrate to transfer the ink is the sum of the top surface area and the shoulder surface area. For high quality printing, it is desirable that the shoulder surface area does not or only minimally contributes with the top surface area in contacting the substrate to transfer the ink. The contribution of the shoulder surface area to the total print area of a particular raised feature can also be influenced by pressure between the substrate and the relief print form during printing. As such, the shoulder surface of raised features should sharply transition from the top surface to the side-wall surface since obtuse or broad transitioning shoulder surfaces readily tend to print. Relief printing forms that have fine features with broad shoulder surface area can result in poor print performance, that is, poor reproduction of the image printed on the substrate. Poor reproduction of printed images can be problematic particularly for printed images that are the combination of fine raised lines and reverses, such as bar codes that are electronically scanned for the contained information. In some instances, printing the shoulder surface area with the top surface area can be further exacerbated by the orientation of the image on the printing form relative to the print direction particularly in rotary printing, i.e., relative to the longitudinal axis of the print cylinder.
It is desirable to eliminate the costs and the time consuming and process steps associated with the preparation of the photographic negative intermediate and transition from analog workflow to digital workflow in the fabrication of solid plates, while avoiding the dot-sharpening effect of the relief features associated with conventional digital exposure in the presence of atmospheric oxygen to create relief features having an analog-like appearance. For high quality printing, the relief printing form should have a relief surface is capable of printing detailed fine highlights in halftone images and/or clean fine text and line graphics without broadening of the shoulder area, and capable of holding solid screen patterning in solid areas for printing solid ink areas with increased density.
So a need arises for a method of fabricating a relief printing form from a photosensitive precursor element that utilizes a digital-like workflow for its ease and simplicity while utilizing washout development that results in the printing form having a relief structure with features necessary for high quality printing.